This invention relates to an electro-optical device having a large number of pixels which is used in the display panels of measuring instruments, instrument panels of cars, personal computers, picture display apparatus, television sets, liquid crystal shutters for printers, etc.
The present invention resides in an electro-optical device of matrix configuration having a large number of row and column electrodes, and is featured in that nonlinear resistance elements made of an amorphous material whose main components are silicon and carbon are formed in series with a layer made of a material having electro-optical effect, one for each pixel including a driving electrode made of that layer, so that large contrast can be obtained even if driven by low voltage, and the power consumption of the device is reduced.
Small-size and light-weight electro-optical devices have been realized using materials, such as liquid crystal, having electrochromism or electro-optical effect. Recently, for the purpose of increasing the amount of information handled in electro-optical devices of this type, attention has been given to a three-terminal active matrix liquid crystal display device made of TFT thin-film transistors, MOS transistors formed on a silicon monocrystal, etc., and to a two-terminal active matrix liquid crystal display device in which each nonlinear resistance element is provided in series with each liquid crystal pixel.
The two-terminal-element active matrix includes a small number of films formed in place as compared with the three-terminal-element active matrix, and thus needs only a reduced number of photo-etching processes; accordingly, its advantages are that a possibility of the device becoming defective owing to dust and the like is small and that the precision of patterning is not necessary to be high; hence, this type of active matrix can be applied to low-cost and large-area electro-optical devices.
The two-terminal active matrix electro-optical device will take one of the following systems, as well known in the art.
(1) MIM (metal insulator metal) system
This system is disclosed, for example, in U.S. Pat. No. 4,413,883, and can be driven by low voltage but has the disadvantage that the number of divisions cannot be increased too much because its current-voltage characteristic is not steep enough.
(2) Ring diode system
This system features every two amorphous Si pin diodes connected parallelly and reversely with each other, as disclosed in U.K. Patent GB 2129183, and its currentvoltage characteristic is comparatively steep. However, it has the disadvantage that the configuration of elements and the manufacturing process are complicated, that is, each pixel needs two diodes and the diodes must be made of three layers (p-layer, i-layer, and n-layer) of amolphous Si.
(3) ZnO varistor system
This system is disclosed, for example, in Japan Patent Publication (Kokai) No. 105285/1980. It has a good nonlinear characteristic, but is defective in that the driving voltage is as high as some tens of volts and an ordinary liquid crystal driving IC cannot be used. Also, it has the disadvantage that this system cannot provide a transmission type display because ZnO of a substrate is opaque.
(4) Back-to-back diode system
This system utilizes the property of a Schottky junction portion between amorphous Si and a metal electrode which shows a nonlinear characteristic, thus its configuration is comparatively simple. However, this system has not yet provided a sufficient nonlinear characteristic for the electro-optical device.
To drive the electro-optical liquid crystal device, the current-voltage characteristic of a nonlinear element film and the allocation of voltage to a liquid crystal layer must be made optimal, and therefor, the resistance and capacitance of the nonlinear element film and of the liquid crystal layer must be selected optimally.
In the prior art, for the electro-optical devices of the MIM system or the like, some compound (e.g. Ta.sub.2 O.sub.5) having a certain specific resistance and a certain dielectric constant was selected as the material of the nonlinear elements, and the resistance of a nonlinear element film was determined by regulating the film thickness. However, if the film thickness is varied to make the film reistance approach a desired value, the capacitance of the nonlinear elements varies also and cannot approach a desired value; on the contrary, if the film thickness is regulated to make the capacitance approach a desired value, the resistance cannot approach a desired value. Thus the prior art could not meet both the current-voltage characteristic of the nonlinear element film and the allocation of voltage to the liquid crystal layer simultaneously.
FIG. 2 illustrates a prior invention which was devised to overcome the foregoing defects and is disclosed in greater detail, for example, in U.S. patent application Ser. No. 784,239 (European Patent Application Publication No. 182484A) and U.S. patent application Ser. No. 863,199 (European Patent Application Publication No. 202092A).
In FIG. 2, reference numeral 7 represents a lower transparent substrate, which is made of ordinary glass. Reference numeral 8 represents a transparent conductive film which is formed by magnetron sputtering of an indium tin oxide (ITO) film. Photoetching is used for the pattern formation. Reference numeral 12 represents a nonlinear resistance layer made of an amorphous material consisting principally of silicon and oxygen, or principally of silicon and nitrogen.
Reference numeral 4 represents a metal electrode which is one of the row and column electrodes. Reference numeral 5 represents a liquid crystal layer having a twist nematic structure. Reference numeral 10 represents an upper transparent substrate, which is made of ordinary glass. Reference numeral 11 represents a transparent conductive film ITO formed on the upper transparent substrate, which is used either as the row electrode or as the column electrode.
Row liquid crystal driving electrodes and column liquid crystal driving electrodes are formed on a substrate and on an opposed substrate, respectively, which are normally 100-1000 in number per group. Each X-Y intersection has a liquid crystal 5 and a nonlinear resistance element 12 formed in place.
In this type of liquid crystal electro-optical device, to provide large-area and high-resolution devices or devices having a large number of pixels while keeping high contrast, the nonlinearity of the nonlinear resistance element 12 must be sufficiently large. Accordingly, where the nonlinear resistance film is made of the silicon nitride film whose composition of silicon is greater than the stoichiometric composition as done in the prior art (see FIG. 2 and curve b of FIG. 4), its nonlinearity is not sufficient, hence, large-area devices made accordingly give only low contrast. Further, because the nonuniformity of distribution of the film thickness of the nonlinear resistance film 12 magnifies with increasing area of the device and the nonlinearity of the nonlinear resistance film 12 is small, this uneven distribution of film thickness results in display flecks. Further, where the non-linearity of the nonlinear resistance element 12 is not sufficiently high, a voltage to be applied across A-C of FIG. 6 must be large in order to make an effective voltage to be applied to the liquid crystal at a selected point large than the saturation voltage of the liquid crystal; hence, the power consumption of the device becomes large.